A power supply controller is conventionally provided, in which a high-power semiconductor switching element such as a power MOSFET is disposed on a current supply line connected between a power source and a load, and which is configured to control the power supply to the load by switching the semiconductor switching element between ON and OFF. In such a power supply controller, it is known that a self-protective function is provided for protecting its own semiconductor switching element. The self-protective function turns off the semiconductor switching element by controlling the potential of the control terminal (e.g., the gate in the case of a MOSFET) of the semiconductor switching element, when an overcurrent (i.e., an abnormal current) has occurred due to short-circuiting in the load, for example. Specifically, as shown in JP-A-2001-217696, for example, a current detecting resistor is serially connected to the load terminal (e.g., the source or drain in the case of a MOSFET) of the semiconductor switching element. The voltage drop on the resistor is detected, and an overcurrent anomaly is determined so as to shut off the semiconductor switching element if the voltage drop is higher than a predetermined level.
A current passing through the semiconductor switching element will vary along a predetermined load line, until the current is stable after start-up of the semiconductor switching element. Therefore, in the case that an overcurrent anomaly is detected based on comparison of a load current, passing through the semiconductor switching element, with a threshold, the problem arises that it may require time before detection when an overcurrent anomaly has occurred, if the threshold is set to a fixed level. For example, FIG. 5 shows the drain-to-source voltage Vds of a power MOSFET and the current Id passing therethrough. In the case that the load is in a normal state, the values of the drain-to-source voltage Vds and the current Id will ideally vary along the load line L0 starting from the point B0 resulting in settlement at the stabilization point A0, while the power MOSFET is maintained ON after the power MOSFET turns ON.
However, in case that an anomaly such as short-circuiting in the load has occurred, the source voltage of the power MOSFET will rise very little after starting from the point B0 on start-up, because the voltage drop in the load is extremely low. That is, the current Id passing through the power MOSFET will rise steeply while the drain-to-source voltage Vds of the power MOSFET varies little. If the threshold is set to a fixed level (as shown by Line L7 in the figure), the level should be determined so as to cover the entire load line. Therefore, in case that short-circuiting occurs immediately after the power MOSFET turns ON as described above, it requires considerable time before the threshold is reached as shown by the line L6. This will result in great power loss in the power MOSFET, and cause delay in protection.
Thus, there is a need in the art to provide a construction capable of detecting an overcurrent anomaly rapidly so as to achieve appropriate protection in a power supply controller having an overcurrent detecting function.